Water Heater does not heat all 6 gallons

I have a 6 gallon Suburban water heater, on the last trip we ran out of hot water much quicker than usual.

When we got home I ran the water heater, made sure it was nice and hot, then measured 3.5 gallons of water before it got cold.

If it wasn't heating at all, or just warm I could understand it, or if we were just blowing through all 6 gallons I could understand that.

But how could it heat only half of the 6 gallon capacity? Maybe an air bubble?

Thanks for your help.